In recent years, hybrid electric vehicles or electric vehicles have been put into practical use from the viewpoint of the global environment protection. The hybrid electric vehicles refer to vehicles that are provided with an engine and a motor as a driving source and that run using one or both of them. Such hybrid electric vehicles and the like are provided with a booster circuit in an electrical power distribution system for the motor. As one of components of the booster circuit, a reactor capable of storing electric energy as magnetic energy is utilized.
The reactor has a coil and a core, in which a closed magnetic circuit is formed in the core by excitation of the coil. As the core, a core constituted by a powder molded product is mentioned. The powder molded product is constituted by press molding multiple composite magnetic particles in which metallic magnetic particles are coated with an insulating coated film. When such a core is used in an alternating-current (AC) magnetic field, energy loss referred to as iron loss arises. The iron loss is generally indicated by the sum of hysteresis loss and eddy current loss. As a technique for reducing the eddy current loss among the above, the technique described in Patent Literature 1 is mentioned. Patent Literature 1 discloses specifying a ratio of the major diameter to Heywood diameter of multiple composite magnetic particles.
In contrast, a current waveform applied to the coil is a waveform in which alternating current components have been added to direct current components. When the direct current components among the above increase, the inductance of the coil decreases. As a result, the impedance decreases, causing problems in that the output decreases or the power conversion efficiency decreases. Therefore, in the reactor, it has been required that a reduction in inductance with an increase in the direct current components is low, i.e., direct current superposition characteristics are favorable. As a technique for improving the direct current superposition characteristics, the technique described in Patent Literature 2 is known. Patent Literature 2 discloses using nonregular-shaped soft magnetic powder having a particle size of 5 to 70 μm.
The powder molded product has been subjected to press molding in a manufacturing process thereof. However, defects, such as strain or dislocation, are introduced by deformation of the multiple composite magnetic particles during the press molding. Therefore, the magnetic coercive force of the powder molded product increases, causing a problem in that the hysteresis loss becomes large. As a measure therefore, it is effective to remove the strain or dislocation introduced into the multiple composite magnetic particles in the press molding process by heat treatment of the powder molded product to thereby facilitate the movement of a magnetic domain wall and reduce the magnetic coercive force of a magnetic core. When the heat treatment temperature is higher, the defects can be sufficiently removed. However, when the temperature is adjusted to an excessive high temperature, the insulating coated film decomposes or deteriorates, which increases the eddy current loss. As a technique for reducing damages to the insulating coated film during the press molding while suppressing the deterioration of the insulating coated film, the technique described in Patent Literature 3 is mentioned. Patent Literature 3 discloses providing a heat-resistance imparting protective film and a flexible protective film to the multiple composite magnetic particles.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2007-129045
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2004-319652
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2006-202956